The research on ferroelectricity in ultrathin films are challenging but needed for state-of-the-art miniaturized electronic devices. In this talk, I will show the emergence of ferroelectricity in two-dimensional single-element matter: black-phosphorus-like group V (P, As, Sb, and Bi) materials. We analyze the structural properties by calculating the electronic structures and phonon dispersions. Our first-principles calculations show that the buckling distortion, which breaks the inversion symmetry, give rise to the in-plane spontaneous polarization in these puckered materials, which can be modulated by the external strains and n-type doping. In addition, the coexistence of ferroelectric and non-trivial topological phase in tensile-strained puckered Bi is also predicted.